Why not pure phosphoric acid for rust removal?

[MetalMonkey]

Just as an addendum to this, I’m just processing the last few bits of rusty box section using a 10% acetic acid (strong vinegar) bath.
I was sceptical to start with but have been very impressed with the results – steel comes out lovely and clean, no mill scale, no rust and none of the problems associated with using hydrochloric acid, it just takes a bit longer.

I was concerned I’d have a lot of issues with rusting after treatment, but it has been fine. It will flash if you don't clean and dry it quickly, but I've had some stood for several weeks and still looks bright.
Here is a before and after, and that isn’t even gnarliest piece I had to clean up.

 

[Ross365]

Just as an addendum to this, I’m just processing the last few bits of rusty box section using a 10% acetic acid (strong vinegar) bath.


View attachment 332249

But this brings us back to citric acid (and the long running thread on that). Citric is a type of acid called carboxylic, as is acetic, but citric is a little more reactive than acetic (and has no smell) so is faster than acetic. Unlike phosphoric, citric does not leave any kind of layer behind, so it offers no protection, but nor does it affect zinc phosphate primers, which I suspect phosphoric may.

 

[vw1]

But this brings us back to citric acid (and the long running thread on that). Citric is a type of acid called carboxylic, as is acetic, but citric is a little more reactive than acetic (and has no smell) so is faster than acetic. Unlike phosphoric, citric does not leave any kind of layer behind, so it offers no protection, but nor does it affect zinc phosphate primers, which I suspect phosphoric may.

This is the part I'm struggling to understand. Some have used phosphoric and let dry and painted directly over the top.

I thought and I maybe wrong but after it has dried it needs to be washed with water. ?

 

[Ross365]

This is the part I'm struggling to understand. Some have used phosphoric and let dry and painted directly over the top.

I thought and I maybe wrong but after it has dried it needs to be washed with water. ?

Yes, this is a question where differences of opinions exist. You "pays your money and takes your choice". I have used phosphoric in the past, but haven't used it for ages, preferring to use a zinc phosphate primer paint (which I'm really happy with) which I assume puts down a layer of phosphate as it reacts with the metal? The makers of the paint do not suggest using any chemical before applying the primer and don't have a phosphoric acid type product in their range.

 

[MetalMonkey]

All these treatments have their uses and their own pros and cons.
But I think you can probably assume that if you are having to use any of them to treat what you are painting, you are already far from having the optimum surface on which to apply your paint system.

i.e. to achieve the best results paint manufacturers are usually going to assume nice clean material, perfectly prepped and paint applied under ideal conditions.
But we all know in the real world you have to make do with the cards you are dealt - in which case anything you can use to improve the situation and remove the rust is going to be a good thing.

 

[Ross365]

Yes, I think you are spot on. Mostly, I'm dealing with my two old cars when I'm dealing with rust, and cleaning up surfaces with a wire brush on a drill or angle grinder, car on axle stands and me underneath. It ain't going to be perfect!

In terms of preparing surfaces, if you want to get serious, the attached is interesting.

 

[Hopefuldave]

I don't think that there is a problem with phosphoric "attacking stuff forever", but it may well be the case that chloride ions can get involved in "chain reactions" which persist and why I never use hydrochloric.

As for wetting out, you might try adding some IPA to the mix as a thinning / wetting agent. I don't think it will get involved in the chemistry of what goes on, but might make the phosphoric easier to apply.

I've used household surfactants (washing-up liquid) with full strength phosphoric, sticks to the metal...

Dave H. (the other one)

 

[Ross365]

I've used household surfactants (washing-up liquid) with full strength phosphoric, sticks to the metal...

Dave H. (the other one)

I would be wary of household products and definitely washing up liquid. Many of them contain salt (sodium chloride) esp the washing up liquid, and so we are back to having chloride ions in play.

 

[charlysays]

This is the part I'm struggling to understand. Some have used phosphoric and let dry and painted directly over the top.

I thought and I maybe wrong but after it has dried it needs to be washed with water. ?

I have done that on stuff I didn't care about much and I did do a test panel on some old rusty 20g steel I had where I painted 45% phosphoric on, let it dry and went straight over with some bilt hamber epoxy mastic which has zinc phosphate in it. There was no loss of adhesion that I could detect vs the EM straight onto the rusted semi shiny steel. Scraping at it with a screwdriver tended to parr off and scuff the epoxy but not peel it off the substrate.

Kurust and jenolite are basically phosphoric acid and neither of them advise rinsing. What seems to work for me is a compromise between the two which is paint it on, keep it moving/ adding more/ wirebrushing until there's satisfactory rust removal, wipe excess off, allow to dry then go over it again with wire wheels and 60 grit DA before hosing down with 99.9% IPA or brake cleaner (tend to use IPA if there is any seam sealer still around as brake cleaner seems to attack it) and then painting. That way, any residual oxides have had a good chance at being phosphated and/ or removed and you still have largely clean, shiny, keyed up steel. Any residual dried acid or salts etc seem to be minimal and haven't caused any issues with coatings even after years. I'm just allergic to painting over anything which looks brown lol, even if it's only in craters.

If I can rinse I do though, but under a car with open seams I don't think rinsing is the best idea and the above method gets rid of enough of the corrosion/ phosphate residues for it not to cause any noticeable issues.

 

[charlysays]

I would be wary of household products and definitely washing up liquid. Many of them contain salt (sodium chloride) esp the washing up liquid, and so we are back to having chloride ions in play.

A good alternative would be cellulose powder as used for papier-mâché. Scola cell is one brand. You can thicken it up as much as you want then.

 

[fractured]

Would the cellulose/phosphoric acid mix/paste have to be removed after treatment/drying? I have never had any dealings with the cellulose powder as papier-mache we used as a kid was old newspapers and some paste.

I noted on the HMG literature that steel should be vacuumed or blasted with clean dry compressed air, so I presume that is saying remove the maximum amount of loose material.

I am really interested in this subject as I have an old land rover and have just started using electrolysis. Due to that process leaving a residue on the steel I am interested in the next stage of the rust treatment. My thoughts were that treating with phosphoric acid may bind any loose material to the steel but would worry that the cellulose would introduce more potentially loose material. This is an issue in this instance as I am looking at the phosphoric as a pretreatment rather than rust removal.

I must admit I had not thought of vacuuming before. I wonder if I could adapt a wire brush to accept a vacuum attachment? That may help to reduce the amount of horid black dust in the air!